Methods using an acknowledgement and methods using a negative acknowledgement have been known as methods for determining whether a receiver has correctly received a signal transmitted by a transmitter or not.
For example, a wireless LAN system uses an acknowledgement (ACK). A receiver in the wireless LAN system transmits an ACK signal only when the receiver has correctly received a signal transmitted by a transmitter.
On the other hand, JP-4110522-B discloses a one-to-one communication system using a negative ACK (NACK). Only one transmitter and only one receiver belong to the one-to-one communication system. Therefore, the transmitter and the receiver in the one-to-one communication system hardly suffer interference from any other wireless apparatus. Accordingly, probability of success in receiving a signal is higher than probability of failure in receiving a signal. The receiver in JP-4110522-B transmits a NACK signal only when the receiver has failed in receiving a signal.
Any reply signal (ACK signal or NACK signal) is a control signal which includes no data. Thus, the throughput of the system deteriorates when the number of times of transmitting the reply signal increases. The throughput of the system can be improved if either an ACK or a NACK is used in accordance with a propagation environment as in the wireless LAN system or the system in JP-4110522-B.
There are plural kinds of reply signals, such as an ACK signal, a NACK signal, a BA (Block ACK) signal, etc. Therefore, the transmitter transmits a signal having a header portion to which information (reply signal type, ReqACKType) indicating that which reply signal should be used is added. The receiver demodulates the header portion of the signal and sends back a reception result using the reply signal specified in the header portion.
When the receiver fails in receiving a header portion of a signal due to deterioration of the propagation environment, the receiver cannot demodulate the reply signal type and cannot send back a reply signal.
When there is no reply from the receiver in the case where the transmitter requests a NACK signal, the transmitter cannot determine whether there is no reply because the receiver has succeeded in receiving a signal, or a reply cannot be sent back because the propagation environment deteriorates.